Laboratory Evaluation for Low Sex Drive
For patients with low libido, a morning serum testosterone level should be measured as the primary laboratory test, especially in men with symptoms or signs of hypogonadism. 1
Primary Laboratory Tests
For Men:
- Morning total testosterone level - Must be collected in the morning when levels are highest 1
- Free testosterone calculation - Using total testosterone, sex hormone binding globulin (SHBG), and albumin concentrations 1, 2
- Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) - To determine if hypogonadism is primary or secondary 1
For Women:
- Free testosterone index - Low free testosterone is significantly associated with decreased sexual desire in women 3
- Sex hormone binding globulin (SHBG) - To calculate free testosterone index 3
Additional Laboratory Workup
Cardiovascular Risk Assessment
- Lipid panel - Low HDL cholesterol is associated with sexual dysfunction 1
- Fasting glucose/HbA1c - Diabetes is strongly associated with sexual dysfunction 1
- Blood pressure measurement - Hypertension contributes to sexual dysfunction 1
Other Hormonal Tests
- Thyroid function tests (TSH, free T4) - Thyroid disorders can affect libido 4
- Prolactin level - Hyperprolactinemia can decrease sexual desire 4
- Estradiol - Particularly important in women 3
Clinical Considerations
Men-Specific Considerations
- Testosterone levels decline with age but symptoms don't always correlate with specific levels 4
- Low testosterone is associated with decreased sexual desire, reduced nocturnal erections, and reduced ejaculate volume 4
- 36% of men seeking consultation for sexual dysfunction have hypogonadism 1
- Low testosterone is associated with increased all-cause and cardiovascular mortality 1
Women-Specific Considerations
- Free testosterone index correlates with sexual thoughts and "need for sex" in women 3
- HSDD (Hypoactive Sexual Desire Disorder) affects approximately 30% of women 5
- The relationship between testosterone levels and sexual function in women is complex and may be confounded by limitations of assays at low concentrations 4
Common Pitfalls to Avoid
- Timing errors: Testosterone should be measured in the morning when levels are highest 1
- Using total testosterone alone: Free testosterone is the metabolically active fraction and may be more clinically relevant 2
- Overlooking cardiovascular risk: ED and low libido may be early markers of cardiovascular disease 1
- Ignoring sleep disorders: Obstructive sleep apnea can contribute to low testosterone and should be screened for in at-risk patients 1, 6
- Missing psychological factors: Depression strongly correlates with sexual dysfunction, especially in women with diabetes 4
Monitoring Recommendations
For patients starting testosterone therapy:
- Monitor total testosterone levels (target: 450-600 ng/dL)
- Monitor hematocrit (discontinue if exceeds 54%)
- Monitor PSA (consider biopsy if >4.0 ng/mL or increases >1.0 ng/mL/year) 6
By following this comprehensive laboratory evaluation, clinicians can identify the underlying causes of low libido and develop appropriate treatment strategies to improve sexual function and quality of life.